Surface acoustic wave (SAW) devices use waves propagated on the surface of an elastic solid for electronic signal processing. A typical SAW device uses a transducer to convert electromagnetic signal waves, which travel at the speed of light, to acoustic signal waves, which travel at speeds on the order of 10.sup.5 less than the speed of light. This substantial reduction in wave length allows designers to implement certain complex signaling processing functions in a significantly smaller space than would be needed for traditional circuit designs. Thus, a SAW device designed to handle complex signal processing functions can offer considerable cost and size advantages over competing technologies. SAW technology is increasingly found in applications such as filters, resonators, oscillators, delay lines, and other similar devices.
SAW devices are typically implemented on a piezoelectric substrate and usually employ interdigital transducers (IDTs) located on the surface of the piezoelectric substrate to generate and detect acoustic waves. An example of an interdigital transducer and its' associated equivalent circuit model is shown in FIGS. 1 and 2 of the accompanying drawings. The geometry of the IDTs (beam width, pitch, number of fingers) on the piezoelectric substrate 101 plays a significant role in the signal processing and frequency response characteristics of a SAW device. The interdigital transducer 100 includes electrode bus bars 102, 104 and electrode fingers 106, 108 extending from each electrode bus bar in an interdigitated configuration. The equivalent circuit model 200 is shown between similar nodes 1 and 2 of the interdigital transducer 100. The pitch determines the frequency of operation of a given transducer and is defined as the finger width added to the space between fingers. The beam width and the number of fingers determine the static capacitance, Co, of the transducers. The beam width is defined as the spacing between, but not including, the electrode bus bars. SAW device designers generally achieve the desired operating frequency response of the device by focusing on the geometry of the IDTs, and by the choice of materials used for the piezoelectric substrate.
Conventional SAW filters have transducers located side by side on common tracks and use acoustic coupling to couple between the transducers, however, each acoustic track must be tuned for the same operating frequency. An improvement over the conventional SAW filter is the ladder filter. Conventional SAW ladder filters are differentiated from conventional non-ladder filters in that the transducers in the ladder design are acoustically uncoupled (staggered), use only electrical coupling, and operate at different frequencies (resonant and anti-resonant) at or near a center frequency, f.sub.o.
SAW ladder filter designs have demonstrated good performance in the areas of wide fractional bandwidth and low insertion loss. However, both conventional ladder filter designs are typically single ended devices where each transducer impedance element requires a different pitch and acoustic beam width for proper resonator coupling. The tolerances on the individual transducer finger pitches in these conventional single ended ladder filters are required to be very high and are difficult and costly to manufacture. With the increased interest and use of differential circuitry in radio communications devices to achieve the advantages of common mode rejection associated with differential filters, it is crucial that small high performance differential selectivity be available.
Hence, there is a need for a differential SAW filter that provides high performance, particularly with regards to insertion loss and selectivity, and which is also easily manufactured.